21663640 has 32 divisors (see below), whose sum is σ = 48906000. Its totient is φ = 8636544.

The previous prime is 21663623. The next prime is 21663647. The reversal of 21663640 is 4636612.

It is a junction number, because it is equal to n+sod(n) for n = 21663599 and 21663608.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21663647) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 14446 + ... + 15874.

Almost surely, 2^21663640 is an apocalyptic number.

21663640 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21663640 is an abundant number, since it is smaller than the sum of its proper divisors (27242360).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21663640 is a wasteful number, since it uses less digits than its factorization.

21663640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1819 (or 1815 counting only the distinct ones).

The product of its (nonzero) digits is 5184, while the sum is 28.

The square root of 21663640 is about 4654.4215537487. The cubic root of 21663640 is about 278.7685717616.

The spelling of 21663640 in words is "twenty-one million, six hundred sixty-three thousand, six hundred forty".